Development of a simulation model of an Air New Zealand aircraft materials remanufacturing system

Abstract

Remanufacturing is a process involving the repair or refurbishment of worn parts. Systems involved with remanufacturing differ from more traditional manufacturing operations in terms of material and work content characteristics. Typical assemblies, for example aircraft engines, are disassembled and worn parts removed for repair or scrap. Parts are subject to variable recovery rates and lead times, while service requirements are unknown in advance and processing times are stochastic. These factors seriously complicate planning and control strategies for firms involved with remanufacturing. Inventory costs associated with remanufacturing are also high, as buffer stocks of serviceable parts are required so that assemblies can be serviced and reassembled by the required date given the uncertainty.

Materials Repair on Air New Zealand's Christchurch Base is a complex remanufacturing system of job shops that primarily supports engine servicing and overhaul. It is also carries out repair and custom manufacturing for other airline sections, as well as for certain external customers. However delays and missed target dates continually occur due to the varying mix and volume of products in process. While it is widely recognized that delays affect both profitability, and the quality of service supplied to customers, the dynamic nature of the remanufacturing operations makes it difficult to anticipate delays or their effects. Development of a decision support tool capable of minimising such delays is worthy of further investigation to improve profitability and enhance customer loyalty. Such a tool could be used to test new management strategies or evaluate production schedules. Discrete event simulation was chosen as the most appropriate way to develop such a tool.

A simulation model of the repair system, named SIMMAT, was developed as a prototype to support with decision analysis. Over 8000 lines of code were written to model the system using SIMSCRIPT II.5®, a specialist simulation language. Analysis of the repair system was conducted by consulting with staff at all levels. SIMMAT effectively consists of structured logic statements based on observations and information supplied by the staff. It processes information related to staffing levels and work received over time to depict how the repair process is likely to react. Processing times are represented using stochastic probability distributions. Outputs include numerical data and animated graphics.

For evaluation purposes SIMMAT was set to simulate Repair Group operations for a recent period of time. It was found that after an appropriate warm-up period SIMMAT's performance could stabilise at a level consistent with normal operation. Simulated workloads were compared against the recollections of experienced staff in the absence of objective records. These staff felt the model gave a credible indication of how the actual system had performed. Two ‘what-if’ scenarios were also investigated that staff should have found reminiscent with their own experiences. The effect and duration of changes under the two scenarios were consistent with the expectations of consulted staff.

On the basis of research presented in this thesis, the simulation model developed has good potential to support scheduling and decision making in remanufacturing systems like Air New Zealand's Material's Repair Group.